Paper 2026/035

Adaptive NIKE for Unbounded Parties

Brent Waters, The University of Texas at Austin, NTT Research

Abstract

This paper presents the first construction of adaptively secure non-interactive key exchange (NIKE) for an unbounded number of parties in the standard model. While prior unbounded protocols were restricted to static security or required random oracles, this work achieves adaptive security in the standard model. The proposed scheme supports an unbounded number of honest and malicious users, as well as unbounded party sizes, while tolerating a bounded number of dynamic user corruptions. The construction is based on sub-exponential indistinguishability obfuscation and sub-exponential fully-homomorphic encryption. A key technical contribution is a new application of what we call a function-extractable hash function. This is a variant of a function binding hash function that enables resilient extraction of properties from maliciously hashed digests. As an additional contribution, we present a compiler in the random oracle model that upgrades any (unbounded) NIKE that does not support dynamic user corruptions at all into a fully adaptive (unbounded) NIKE that supports an unbounded number of dynamic corruptions. This compiler is completely generic, does not introduce any additional assumptions, and does not rely on sub-exponential hardness.

@misc{cryptoeprint:2026/035,
      author = {Shafik Nassar and Brent Waters},
      title = {Adaptive {NIKE} for Unbounded Parties},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/035},
      year = {2026},
      url = {https://eprint.iacr.org/2026/035}
}